The present invention relates to a measuring unit intended to be used in a bending machine, in particular in a bending machine for bending conductors for superconductive coils, to measure the bending radius and the forwarding, in particular the forwarding after bending, of the workpiece under bending, as specified in the preamble of independent claim 1. Such a measuring unit is known from EP 0 767 016 A2.
It is known that a workpiece under bending in a bending machine undergoes changes in length (which are typically in the sense of an increase in the length of the workpiece, but which may also be in the sense of a reduction in the length of the workpiece) as a result of the plastic deformations to which the workpiece is subject during the bending process. It follows that in all those applications where it is required to measure in real time, as much precisely as possible, the forwarding of the workpiece, in order to be able to control the bending process taking into account the actual measure, it is not sufficient to provide for measuring device arranged to measure the forwarding of the workpiece upstream of the bending machine, but it is necessary to provide for measuring devices that are also placed downstream of the bending machine, whereby the actual forwarding of the workpiece after bending can be measured. An application where the control of the forwarding of the workpiece is of the utmost importance is for example the manufacturing of superconductive coils, such as those intended to be used in nuclear fusion reactors. These coils are in fact manufactured by bending very long conductors, for example with a length in the order of hundreds of meters, and therefore an inadequately precise control of the forwarding may result in exceeding the prescribed dimensional and geometric tolerances and hence in having to discard the thus manufactured coils, with—as is evident—very significant economic damages.
In addition to the forwarding, another fundamental parameter of the bending process is the bending radius. It is therefore advantageous to provide a measuring unit that allows to measure both the forwarding, in particular the forwarding after bending, and the bending radius of the workpiece that is being bent in a bending machine.
According to the solution disclosed in the above-mentioned prior art document EP 0 767 016 A2, the measuring unit comprises: a support structure; a pair of measure rollers mounted on respective movable bodies so as to rotate idly about respective axes of rotation parallel to each other and perpendicular to the bending plane; a pair of encoders associated each to a respective measure roller to measure the angular position of each measure roller about the respective axis of rotation; a pair of linear guides for guiding the movable bodies along a straight direction perpendicular to the axes of rotation of the measure rollers; a pair of springs interposed each between the support structure and a respective movable body to urge the measure roller carried by this body against the extrados surface and against the intrados surface of the workpiece, respectively; a linear sensor adapted to measure the distance between the extrados and intrados surfaces of the workpiece; and a pair of guide rollers that are mounted at the ends of respective arms of the support structure oriented parallel to the linear guides and arranged on opposite sides of the linear guides and that are kept continuously in contact with the intrados surface of the workpiece to ensure the correct positioning of the support structure, and hence of the linear guides, relative to the workpiece. The fact that the guide rollers are kept in contact with the intrados surface of the workpiece is ensured by the elastic force applied by the spring that acts on the movable body carrying the measure roller in contact with the extrados surface of the workpiece being greater than the elastic force applied by the other spring. In other words, the measure roller in contact with the extrados surface of the workpiece acts also as counter roller that, acting on the workpiece on the opposite sides with respect to the two guide rollers, ensures the correct positioning of the support structure relative to the workpiece.
The main drawback of this known solution is that the normal force applied by the measure roller in contact with the extrados surface of the workpiece causes the latter to flex, at least elastically, which results in a change in the curvature of the workpiece (the lower the flexural modulus of the workpiece, the higher the curvature change) just at that section where measuring is carried out by the measure rollers. This results in a reduction in the measurement precision.